Method for concentrating dilute nitric acid



Sept 4. 1956 w. J. coNGDoN ETAL 2,761,761

METHOD FOR CONCENTRATING DILUTE NITRIC ACID Original Filed March 2O ATTORNEY.

METHOD FOR CONCENTRATIN G DILUTE NITRIC ACID Congdon, Hopewell, and Samuel W. Grossmann, Petersburg, Va., assignors to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York n Originalapplication March 20, 1951, Serial No. 216,592, Y now Patent No. 2,665,195, dated January 5, 1954. Divided and this application October 19, 1953, Serial No. 386,770

1y Cla-im. (Cl. 23-160) This invention relates generally to the production of chlorine from HC1 involving the use of 'concentrated nitric acid with the formation of dilute nitric acid. This application is a division of my copending application Serial No. 216.592 led March 20, 1951, now Patent No. 2,665,195, and refers more particularly to a new and improved process for concentrating dilute nitric acid.

In many chemical` plants chlorine is reacted with various organic compounds to produce desired chlorinated compound together with substantial quantities of unwanted by-product hydrochloric acid. This by-product .hydrochloric acid in some instances is largely wasted, and not only represents a'loss of resources but also presents a waste disposal problem. In addition, because of chlorine shortage additional chlorine required by the chemical manufactures is not always available. Practically allof the chlorine commercially manufactured today involves large scale production with large capital investment and further produces from the process other products such as caustic or nitrate salts. :Some chemical manufacturers do not have the means or the desire to enter into large scale chlorine manufacture and do not wish to be troubled with the sale or other disposition of caustic or nitrate products accompanying such manufacture.

One object of the present invention is to provide an etiicient continuous Vprocess for converting HCl into chlorine. t

Another object of this invention is to provide a process for producing as thesole product of the process high yields of substantially pure chlorine.

- A further object of this invention is to provide an economical process for :producing chlorine from HCl with the consumption of only minor amounts of nitric acid.

Further objects and advantages will be apparent from the following description and accompanying drawing.

A preferred method of' carrying out that'portion ofV our process for etecting conversion of HC1 into chlorine involves a two-stage cyclic operation wherein HCl is reacted in a rst zone with eiuent nitric acid from a second "zone to produce as primary reaction products gaseous chlorine and nitrosyl chloride and a more dilute nitric acid, discharging the more dilute nitric acid from the first zone, passing the gaseous chlorine and nitrosyl chloride into a second zone, introducing nitric acid of at least 75 weight percent concentration into the second zone to react with the nitrosyl chloride to produce additional chlorine, `nitrogen dioxide and water, discharging the mixture lof-.gaseous nitrogen dioxide and chlorine from the second 1zone,` and returning nitric acid diluted by the water of reaction as nitric acid etliuent from the second zone to the rst zone.

The NO2-C12 mixture from the second zone is separated into chlorine which is discharged from the system as the productof the process, and the separated N204 reacted with the dilute nitric acid withdrawn from the trstzone to .produce therefrom a concentrated nitric acid containing at least 75% by weight HNOs, and the thusv concentrated nitric acid recycled to the second zone for' conversion ofthe nitrosyl chloride into chlorine and nitro;l

gen dioxide.

OurY preferred method of reacting N204 with dilute nitric'` acid to produce therefrom nitric acid having a concentration of at least 75% by weight HNOa involves passing N204 together with ,nitric acid cocurrently downwardly through a heated chamber countercurrent to an upward stream of air, withdrawing concentrated nitric acid of at least 75% byweight HNOa from the bottom of the chamber, passing the mixture of air and N204 released from the top of the chamber upwardly through an ab? sorber countercurrent to a downward ilow of dilute .nitric acid, returning concentrated nitric acid from the bottom` of the absorber to the top of the chamber for cocurrent downward ow with N204 countercurrent to the rising stream of air therein, passing the uncondensed gases from the absorber comprising a mixture of nitrogen, N204 and unreacted oxygen through a cooler toefect condensation of the N204, returning the condensed N204 to the chamber for cocurrent flow with nitric acid downwardly countercurrent to the stream of air, passing the residual gases'comprising primarily nitrogen, unreacted oxygen and a minor amount of N204 leaving the cooler through a scrubber countercurrent to a portion of cold concentrated nitric acid withdrawn from the bottom of the chamber thereby removing N204 from the residual gases, returning the concentrated nitric acid after passage through the scrubber to the chamber for cocurrent flow with N204 countercurrent to the stream of air and discharging the nitrogen, oxygen gases substantially free from N204 from the scrubber into the atmosphere.

The accompanying drawing is a diagrammatic ow sheet illustrating the process of the present invention.

Referring to the drawing, hydrogen chloride charging v stock generally admixed with water, as for example 90% HCl-10% water, is introduced in the form of a gas or liquid through line 1 into HC1 digester 2 at a point below the center of the digester. Due to the corrosive naturelof hydrochloric and nitric acids undergoing reaction, digester 2 is a stoneware and acid-proof cement vessel containing sections of ring packing in the base and sections of troughs in the top, all of an acid-resistanttype. Nitric acid of 60-70 weight percent concentration, preferably 65-68%, enters the top of digester 2 through'line 3 and flows down countercurrent to ascending HC1, H20 and reaction products. The vapors leaving the top of digester 2 through line 4 are a mixture of C12 plus NOCi with some HN03 and H20. Liquid discharged from the base of digester 2 through line 5 is nitric acid more dilute than ythe nitric acid introduced into the top of digester 2 through line 3, generally about Ll5-55% HNOa. A steam coil 6 in the base of digester 2 provides suicient heat to accelerate the reaction between HCl and HN03, roughly a temperature of about C. at the top to about C. at the base of digester 2 will be adequate for this purpose. High superatmospheric pressures on digester 2 are unnecessary; low superatmospheric pressure of the order'of 3 p. s. i. gauge were found satisfactory. Chloride concentration in the nitric acid discharged through line 5 should be low in order not to introduce a corrosion problem in the nitric acid concentration equipment. Steam coil 6 serves the additional function of stripping the exit nitric acid to reduce the chloride conf centration to a point where corrosion of stainless steel is not excessive.

NOCl oxidizer 7 is a stoneware, acid-proof cement vessel containing packing similar to HCl digester 2. Nitric acid of 75-85% concentration, or higher enters the oxidizer 7 through line 8 above the ring-packed sections. The

nitric acid prior to entrance into oxidizer 7 is desirably Iatented Sept. 4, 1956 slightly preheated toform a mixture of liquid and vapor by passing the nitric acidV through line 9 into HN03 vaporizer 11 wherein it passes in indirect heat exchange with steam and thencel discharges through line 8 into oxidizer 7 thereby eliminating the necessity for a heater in oxidizer 7, the necessary control being obtained by the amount of acid vaporized in HNOs vaporizer 11. The NOCl vapor from line 4 passes upwardly through oxidizer 7 in intimate contact with the downwardly flowing concentrated nitric acid to effect a reaction therebetween to produce C12 and N204, the reaction being nearly complete by the time the vapor reaches the HN03 feed point, i. e. the entrance of line 8 into vessel 7. The troughpacked section in oxidizer 7 above feed line 8 functions as a nitric `acid concentrating section and serves to eliminate water from the vapor leaving the oxidizer through line 12. The liquid on the top plate of oxidizer 7 will be essentially 100% HNOa with some dissolved N204 and C12.

The vapor released from the top of chamber 7 through line 12 is a mixture of N02, N204, and nitric acid (100%). The conditions maintained in NOCI oxidizer 7 are milder than those in HC1 digester 2, the former having a top temperature of about 65-70 C., a bottom temperature of approximately 100 C. and a pressure of a few pounds gauge.

Vapors from line 12 are cooled to about 40 C. by indirect heat exchange with Water in HN03 condenser 13 to condense out as much HNOs as possible and to reduce the load on the refrigeration system. Nitric acid condensate from condenser 13 is returned through line 14 and ows down NOCI oxidizer 7 as reflux. Uncondensed vapor from condenser 13 enters drying column 15 through line 16 where the last traces of HNOs are removed by scrubbing the vapor with liquid N204--Cl2 introduced into the top of column 15 through line 17. The drying column 15 is a ceramic and acid-proof cement tower containing a plurality of baffle plates to insure intimate contact between the vapors and downflowing liquid. The liquid accumulating in the bottom of drying tower 15 composed of HNOs, N204 and C12 is returned as reilux through line 18 to the top of oxidizer 7. The temperature maintained in the top of column 15 is about 10 C. and at its base approximately 50-70 C. The liquid N204-C12 is introduced through line 17 into the top of drying column 15 at a low temperature about 20 C. to assure effective removal of moisture and HNOa from the vapors introduced therein.

N204-Cl2 dried vapor from the top of column 15 ows through line 19 into oxidation condenser 21 which is a heat exchanger wherein the vapor is condensed by a refrigerant, for example boiling CC12F2. N204- C12 condensate discharges through line 22 into weir box 23 from whence a portieri ows through line 17 into drying column 15 as previously described and another portion is directed through line 24 into separation column 25 wherein N204 and C12 are fractionated into substantially pure C12 at the top and substantially pure N204 at the base. Separation column 25 may be a conventional bubble-cap column operating with the top temperature of about C. and a base temperature of about 55 C. and a rellux ratio of approximately 1.5. Fractionation in column 25 is better etfectuated at superatrnospheric pressure, a pressure of 50-60 pounds may be readily maintained on column 25 by elevating oxidationl condenser 21 sufficiently above column 25 so that the column of liquid in line 24 due to the differential in height will impress a superatrnospheric pressure on column 25. Heat for effecting fractionation of the N204- C12 mixture may be supplied by a steam coil 26 disposed in the base of tower 25. Substantially pure chlorine vapor released from the top of tower 25 through line 27 is cooled and condensed in separation column condenser 28 by indirect heat exchange with a refrigerant such as CClzFz and the condensate therefrom flows through line 29 into Weir box 31 with a'portion of the condensate returning through line 32 into the top of column 25 as reflux and the remainder of the chlorine discharging -through line 33 as an end product of the system. Since the N204 discharging from the bottom of column 25 through line 34 will be used to produce nitric acid of about 80% concentration for recycle by reaction with water in the dilute nitric acid from HC1 digester 2 it should desirably be substantially free of NOCI. Removal of NOCl may be effected by withdrawing through line 35 a small side stream Vcontaining NOCl from column 25 and returning the side stream through line 3 to HC1 digester 2 wherein it is converted to additional chlorine and N204.

Referring again to HCl digester 2, the dilute nitric acid, about concentration, from the base of digester 2 flows by gravity through line 5 to nitric acid tank 36 wherein the acid in the tank is cooled to about 50 C. by conventional means not shown in the drawing. Acid from tank 36 is forced by pump 37 through line 38 into nitric acid concentrator 39, a conventional bubble-cap column, maintained at subatmospheric pressure about 24-26 inches Hg vacuum. Connected to the bottom of column 39 by conduits 41 and 42 is the usual reboiler section 43 which supplies heat for vaporization of the water from the nitric acid. Water removed through line i4 from the top of concentrator 39 is condensed and vacuum obtained in a barometric conventional jet condenser designated generally by numeral 45. A small amount of water, preferably steam condensate, is introduced as reux through line 46. As a result of the vacuum distillation in concentrator 39 the 50% acid is concentrated to abouty 6065% nitric acid which is withdrawn from the bottom of the reboiler section 43 through line 47, cooled in nitric acid cooler 48 and then passed through line 49 into nitric acid surge tank 51.

Although theoretically there is no net consumption of nitric acid in the process since the N204 resulting from reaction of HNO3 and HCl is converted in the system to nitric acid and recycled for further reaction, nevertheless, there is a small loss of nitric acid attendant the operation as is common in most chemical processes. A convenient place for adding make-up HN03 to maintain l equilibrium conditions in the system is surge tank 51 into which make-up nitric acid is added through line 52. The

60% nitric acid in tank 52 is forced through line 53 by'v pum-p 54 into the top of N204 absorber 55, a conventional plate type absorber, wherein nitric acid owsvdownwardly countercurrent to an upward flow of a mixture of vaporized N204 and air entering the bottom of absorber 55 through line 56. The acid forming reaction may be expresesd by the following equation:

The liberated N0 will be partially oxidized in the column by oxidation in the air to regenerate N204 as follows:

The chemical equilibrium in the first reaction is such that a very high ratio of N204:NO in the gas is necessary to produce nitric acid. The rate of oxidation of NO as shown by the second equation is very slow under the low pressure of NO which is required by the equilibrium.

vTherefore, an excess of N204 is maintained inthe gas phase by recirculating N204 through column 55. The

absorption system which includes N204 absorber 55 is4 operated under superatrnospheric pressure of the order of p. s. i. gauge to increase the oxidation rate and fa-l cilitate later recovery of the excess N204 by condensationr This mixture ofv gases flowstion of a large fraction of the N204 in the gases which condensate is returned through line 59 to N204 vaporizer 61. Most of the N leaving absorber 55 will be oxidized to N204 in condenser 58.

The residual gas from condenser 58 enters the Vent scrubber 62 through line 63 wherein it is scrubbed in the packed column with a countercurrent flow of refrigerated 80% HN03 at about 5 C. entering through line 64, to remove th remaining N204 and to eect inal oxidation of any N0 present. The scrubbed gases consisting essentially of nitrogen and unreacted oxygen are vented through line 65 to the atmosphere. The nitric acid and absorbed N204 from the bottom of scrubber 62 at about 30-35 C. are returned through line 66 to N204 vaporizer 61.

Vaporizer 61 is a lm type unit designed so that entering liquids will tlow down as a film over the inside walls of externally steam heated vertical tubes in the vaporizer countercurrent to air which is introduced through line 67 at the bottom of vaporizer 61. The liquids entering vaporizer 61 are N204 withdrawn from the bottom of separation column 25 through line 34 and forced by pump 68 through line 69 into the top of vaporizer 61; nitric acid of about 80% concentration from the bottom of absorber 55 through lines 711 and 72; N204 from condenser 58 through lines 59, 73 and 72; and nitric acid containing dissolved N204 from the bottom of vent scrubber 62 through lines 66, 73 and 72. As the mixture of liquids composed primarily of nitric acid and N204 entering the top of vaporizer 61 tlows downwardly through the steam heated tubes there is vaporized from the mixture N204, leaving liquid nitric acid of about 80% concentration. The air introduced through line 67 into the bottom of vaporizer 61 strips the downowing acid of residual dissolved N204 which acid iinally discharges from vaporizer 61 at a temperature of about 100-140o C. through line 74. Air and gaseous N204 are released from the top of vaporizer 6l through line 56 into the bottom of N204 absorber 55. Approximately 80% of this N204 is recycled. As previously mentioned, in order to produce nitric acid at a reasonable rate by reaction of N204 with H20 it is necessary to maintain a very high ratio of N204 to N0. This we accomplish by recycling at least 21/2 parts of N204 for every part of N204 introduced into absorber 55 as makeup.

Nitric acid of about 80% concentration withdrawn from the bottom of vaporizer 61 through line 74 is cooled with water in cooler 75 and the cooled nitric acid directed through line 76 to surge tank 77. A portion of the nitric acid in tank 77 is returned by pump 7S through line 79, refrigerator 81 wherein the nitric acid is cooled to a temperature of about C. and thence the refrigerated nitric acid sprayed into the top of vent scrubber 62 through line 64. Another portion of the nitric acid from tank 77 is withdrawn through line 82 and forced by pump 83 through line 9 into HNOS vaporizer 11 and thence through line 8 into NOCl oxidizer 7.

From the foregoing description it will be apparent that our process acomplishes conversion of HC1 into substantially pure chlorine in a closed cycle with the consumption of only a minor amount of nitric acid and without the productiton of any by-products.

A specic example for practicing the process in accordance with the present invention is as follows. Hereinafter', unless otherwise stated, all quantities will be expressed in terms of rate of net tons per 24 hours.

Hydrochloric acid (90% HCl-10% water) at the rate of 25.75 tons HC1 is introduced into an HC1 digester maintained under 3 p. s. i. gauge pressure with a top temperature of 100 C. and a base temperature of 120 C. As a result of reaction with 67.7% by weight nitric acid (51.5 tons HNOS-24.5 tous H20) there is evolved from the HCl digester 16.7 tons C12 and 15.25 tons NOCI 6 which is introduced into an N001 oxidizer. 67.7% nitric acid utilized in the HC1 digester is a product from NOCl oxidation. A 52.5% nitric acid (36.5 tons HN03-33 tons H20) is withdrawn as a separate stream from the HC1 digester and directed to the HN03 concentrator for removal of moisture.

In the NOCI oxidizer the C12 and NOCI from the HC1 digester are reacted with 80% nitric acid (80.75 tons HNOS-20.25 tons H20) to produce 25 tons C12 and 32.5 tons N204. This mixture of C12 and N204 is separated by fractional distillation into about 25 tons C12 which is discharged from the system as the product of the process and about 32.5 tons of N204 (equivalent to 44.25 tons HN03) which is directed to a N204 absorption systern for production of 80% NHOs by contact with dilute nitric acid.

The 'dilute nitric acid just referred to is obtained by evaporating in a vacuum still 7 tons of moisture from the 52.5% nitric acid obtained from the HCl digester to produce thereby 62.5 tons of 58.5% HNOa (36.5 tons HNOS-26.0 tons H20). By contacting the 58.5% HN03 with the N204 in the presence of air in the N204 absorption system there is produced 101 tons of 80% nitric acid which is returned for oxidation of the NOCI in the NOCI oxidizer thereby completing the cycle. 0f course, a small amount of make-up HNOa` is added to the system to compensate for the usual losses in such operation.

Although certain preferred embodiments of the invention have been disclosed for purpose of illustration, it will be evident that various changes and modications may be made therein without departing from the scope and spirit `of the invention.

We claim:

A method of reacting N204 with dilute nitric acid to produce therefrom nitric acid having a concentration of at least by weight HNOa which comprises passing N204 together with nitric acid cocurrently downwardly through a separate and distinct heated chamber countercurrent to an upward stream of air, withdrawing concentrated nitric acid of at least 75% by weight HNOS from the bottom of the chamber, passing the mixture of air and N204 released from the top of the chamber, upwardly through a separate and distinct absorber countercurrent to a downward ow of dilute nitric acid, passing concentrated nitric acid from the bottom of the absorber to the top of the chamber for cocurrent downward flow with N204 countercurrent to the rising stream of air therein, cooling the uncondensed gases from the absorber comprising a mixture of nitrogen, N204 and unreacted oxygen in a separate and distinct condenser to effect condensation of the N204 in the ratio of at least 21/2 parts condensate for every part of nitrogen dioxide from an external source, returning the condensed N204 to the heated chamber for cocurrent flow with nitric acid downwardly countercurrent to the stream of air, scrubbing residual gases comprising primarily nitrogen, unreacted oxygen and a minor amount of N204 with cold concentrated nitric acid withdrawn from the bottom of the chamber n a separate and distinct scrubber thereby removing N2O4 from the residual gases, passing the concentrated nitric acid after scrubbing of the residual gases to the heated chamber for cocurrent downward flow with N204 countercurrent to the stream of air, and discharging the scrubbed gases substantially free from N204 from the system.

References Cited in the le of this patent UNITED STATES PATENTS 2,028,402 Luscher Jan. 21, 1936 2,046,162 Handforth et al. June 30, 1936 2,088,057 Handforth July 27, 1937 2,185,580 Beekhuis Jan. 2, 1940 2,665,195 Congdon et al. Ian. 5, 1954 

